1. Field of the Invention
The present invention relates to devices that yield maximum available power density and beam brightness while combining outputs of semiconductor diode lasers into a substantially rectangular shaped spot.
2. Information Disclosure Statement
Semiconductor diode lasers have distinct advantages over classical lasers including smaller size, lower power consumption, and lower maintenance costs. Employing inexpensive semiconductor laser arrays would be preferable for many industrial applications since these lasers offer simple alignment and can easily be substituted for service repair. Such diode lasers could replace conventional laser sources for welding, cutting and surface treatment, or could be used for pumping solid state lasers and optical fiber lasers or amplifiers.
The necessary power for these applications exceeds the power available from a single low-cost diode laser because both the power density on the facet and the heat generated by the diode limits the output power. Therefore, to obtain higher output power, diode laser devices usually consist of several emitters or emitter groups that are combined to form an array of emitters on one or more substrates. Higher output power is attained with this set up, but the effective width of the combined beam also increases, thus making it difficult to focus a beam to a required spot size and divergence.
Suitable optics must be employed to effectively combine highly divergent semiconductor diode laser outputs. Ideally, these optics would combine the outputs into a substantially circular or square shape, which can be focused to a small spot with low divergence to provide the maximum power density and beam brightness available at a work or treatment site. In practice, however, it is difficult to provide maximum power density and beam brightness when combining diode laser outputs.
The maximum transmittable power density and beam brightness that may be available at a work or treatment site is limited to the original beam quality. A measure of beam quality is beam propagation parameter, M.sup.2, which can be calculated and analyzed to determine how well a laser beam may be focused. Generally, the highest quality beam is associated with the highest focusability to the smallest spot size, which corresponds with the highest power density. FIG. 1 shows the variables used to determine M.sup.2, which is directly related to the product of a beam's minimum near-field diameter, W, and beam divergence angle, .theta., in the far field for a specific emission wavelength, .lambda.: EQU M.sup.2 =.pi.W.theta./4.lambda. (1)
Laser beams with M.sup.2 =1 are ideal, and larger M.sup.2 values indicate decreasing focusability of a laser beam; M.sup.2 values less than 1 are unattainable.
The near field is the region at, or very close to the output aperture of the diode laser emitter, which is characterized by disordered phase fronts, and is often called the Fresnel zone. In the near field, shape, size, profile and divergence can vary rapidly with distance along the beam path. The extent of the near field depends on the laser type and for a highly divergent source such as a diode laser, it can be as short as a few microns from the output facet. In contrast, the near field of an excimer laser might be many meters.
At longer propagation distances from the laser, the phase fronts become ordered, leading to stable beam characteristics This is known as the far field, or Fraunhofer zone. A very rough approximation of the distance to the onset of the far field region can be obtained by taking the square of the beam's minimum near field diameter, W, divided by the wavelength, .lambda.: EQU F=W.sup.2 /.lambda. (2)
For example, for a typical HeNe laser having a circular output (W=1 mm, .lambda.=632.8 nm), the distance to the far-field begins at about 1.5 m, while for a typical YAG laser having a circular output (W=10 mm, .lambda.=1064 nm), the far field distance begins nearly 100 m from the source.
The quality of a diode laser beam is typically examined with respect to the fast axis, which is the high divergence axis, perpendicular to the pn-plane of a semiconductor diode, and with respect to the slow axis, which is the lower divergence axis, parallel to the pn-plane of a semiconductor diode. FIG. 2 shows that emitted laser beam 24 from semiconductor laser diode 21 propagates along the z-axis and diverges rapidly along the y-axis, termed the fast axis, which is along the minor axis of diode laser stripe 23. Concurrently, emitted laser beam 24 diverges slower along the x-axis, termed the slow axis, which is along the major axis of diode laser stripe 23. Near the source, emitted laser beam 24 is elliptically shaped with the x-axis being the long axis. The minimum near field diameter, W, is therefore different along the fast and slow axes. For M.sup.2.sub.slow calculations, W is assumed to be the effective diameter along the major axis, or x-axis, and for M.sup.2.sub.fast calculations, W is assumed to be the effective diameter along the minor axis, or y-axis. Since the effective diameter, W, is so much larger along the slow axis compared to the fast axis, M.sup.2.sub.slow typically are larger than M.sup.2.sub.fast, indicating that beam quality is greater along the fast axis. M.sup.2 for a beam is equal to the square root of the product of the M.sup.2 values for both axes, i.e. M.sup.2 =(M.sup.2.sub.fast M.sup.2.sub.slow).sup.1/2.
As emitted laser beam 24 propagates away from the source, it diverges more rapidly along the y-axis than the x-axis. After some distance, laser beam 24 will be circular for an instant, and thereafter, the long axis of the ellipse becomes the y-axis. Generally, for many high power laser diodes, the fast axis diverges at about 40.degree. and the slow axis diverges at about 20.degree..
Adding diode laser emitters to form an array leads to an M.sup.2 value for the "enveloped" beam where "enveloped" is used to describe the combined beams of the diode laser emitters, and is given by: EQU M.sup.2.sub.enveloped &gt;.SIGMA. M.sup.2.sub.n (3)
where n is the number of emitters. M.sup.2.sub.enveloped increases as the effective width of the "enveloped" beam increases, and is greater than the sum of the M.sup.2 values for the individual beams because the effective width includes the space between the emitters. It thus would be advantageous to increase beam quality by employing suitable optics to combine the beams and limit the effects of the space between the emitters. If the beams can efficiently be combined, M.sup.2.sub.enveloped can be decreased, and therefore, greater power density and beam brightness is available at a work or treatment site.
The maximum power density and beam brightness available at a work or treatment site is based upon the original "enveloped" beam without any free space between the emitters. In other words, the highest quality "combined" beam (M.sup.2.sub.minimum) that may be obtained is equal to the sum of the M.sup.2 values of the diode laser beams. This maximum beam quality is hereinafter referred to as M.sup.2.sub.min.
Most gas or solid state lasers emit beams with a divergence angle of about a milliradian, meaning that they spread to about one meter in diameter after traveling a kilometer. Semiconductor lasers have a much larger beam divergence and require suitable optics to reshape the beam and limit the divergence. However, according to Krivoshlykov et al. (U.S. Pat. No 5,751,871), these optics may decrease the focusability of the beam, and further limit the beam quality due to large aberrations of the system resulting from large non-paraxial angles of the laser beam rays, mode mismatches, and tight alignment tolerances. A diode laser device that could simply maintain beam quality would thus be advantageous.
If beams from multiple diode lasers can efficiently be combined, M2enveloped can be minimized within the shaping optics to M.sup.2.sub.mim, and laser energy can be tightly focused to maximum high power densities for welding, cutting and surface treatment. Additionally, decreasing M.sup.2.sub.enveloped to M.sup.2.sub.min, yields maximum transmittable laser brightness for use in pumping solid state lasers and optical fiber lasers or amplifiers.
Various methods have been previously employed to combine the outputs of multiple diode laser emitters. Neuberger et al., in U.S. Pat. No. 5,688,903, describes the state of the prior art which simply comprises coupling radiation from each diode laser emitter into a single optical fiber, and the fiber is then bundled together with other similar fibers. This prior art is efficient in power through-put, but inefficient in maintaining beam quality because the free space of the optical fiber is too large compared to the laser beam, and thus the energy density dissipates as it fills the free space while propagating through the fiber. Products are now available from companies like Opto Power or CeramOptec Industries Inc. that utilize this round optical fiber coupling with diode arrays operating with different geometry and wavelengths.
Additionally, in an alternative embodiment, Neuberger et al. (U.S. Pat. No. 5,668,903) teaches the beam combining device displayed in FIG. 3. Radiation from diode laser emitters 301-304 propagates through flat surface 310 and is reflected and focused by shaped side surfaces 306-309 in the direction parallel to the axis of optical fiber 305 which collects radiation from diode laser emitters 301-304. These state of the art systems can be very complicated and expensive. Moreover, these systems still make it difficult to reach the goal of M.sup.2.sub.min. Thus, to effectively take advantage of the diode laser and address one or more of the above problems, there is a need for a diode based laser system that is small, scalable, inexpensive, and capable of transmission of tightly focusable radiation.